Root Aeration and Respiration in Young Mangrove Plants (Avicennia marina (Forsk.) Vierh.)

The roots of young plants of Avicennia marina (Forsk.) Vierh.grown under simulated tidal conditions were harvested so asto obtain the entire root system. The roots were subdividedand weighed and subsamples taken for manometric determinationof respiration rates at different temperatures. The supply capacityof the above-ground portion of the root system was determinedand the results compared in terms of supply and demand. Theoxygen consumption rate of the roots at 15°C was found tobe 1·69±0·07 µmol kg^–1 s^–1for cable roots and 3·27±0·12 µmolkg^–1 s^–1 for fine roots. The Q₁₀ for respirationwas 2·55 for oxygen consumption in both fine and cableroots, and for carbon dioxide production was 2·66 forfine roots and 3·04 for cable roots. The respiratoryquotient varied with temperature but was less than unity. Concentrationdifferences of between 1·8 mol m^–3 and 3·4mol m^–3 between the inside of root and the air were sufficientto permit aeration of the root system by diffusion alone, andthe aerenchyma contained sufficient oxygen to maintain aerobicconditions while the roots were covered with water. The effectof tide and seasonal temperature change on gas exchange, togetherwith the possibility of some form of carbon dioxide fixationwithin the root, are examined and the implications of theseeffects on growth and development are discussed. Key words: Mangrove, root aeration, respiration, aerenchyma     

Root Distribution, Growth, Respiration, and Hydraulic Conductivity for Two Highly Productive Agaves

Cultivated Agave mapisaga and A. salmiana can have an extremelyhigh above-ground dry-weight productivity of 40 Mg ha^–1yr^–1. To help understand the below-ground capabilitiesthat support the high above-ground productivity of these Crassulaceanacid metabolism plants, roots were studied in the laboratoryand in plantations near Mexico City. For approximately 15-year-oldplants, the lateral spread of roots from the plant base averaged1.3 m and the maximal root depth was 0.8 m, both considerablygreater than for desert succulents of the same age. Root andshoot growth occurred all year, although the increase in shootgrowth at the beginning of the wet season preceded the increasein growth of main roots. New lateral roots branching from themain roots were more common at the beginning of the wet season,which favoured water uptake with a minimal biomass investment,whereas growth of new main roots occurred later in the growingseason. The root: shoot dry weight ratio was extremely low,less than 0.07 for 6-year-old plants of both species, and decreasedwith plant age. The elongation rates of main roots and lateralroots were 10 to 17 mm d^–1, higher than for various desertsucculents but similar to elongation rates for roots of highlyproductive C₃ and C₄ agronomic species. The respiration rateof attached main roots was 32 µmol CO₂ evolved kg^–1dry weight s^–1 at 4 weeks of age, that of lateral rootswas about 70% higher, and both rates decreased with root age.Such respiration rates are 4- to 5-fold higher than for Agavedeserti, but similar to rates for C₃ and C₄ agronomic species.The root hydraulic conductivity had a maximal value of 3 x 10^–7ms^–1 MPa^–1 at 4 weeks of age, similar to A. deserti.The radial hydraulic conductivity from the root surface to thexylem decreased and the axial conductivity along the xylem increasedwith root age, again similar to A. deserti. Thus, although rootsof A. mapisaga and A. salmiana had hydraulic properties perunit length similar to those of a desert agave, their highergrowth rates, their higher respiration rates, and the greatersoil volume explored by their roots than for various desertsucculents apparently helped support their high above-groundbiomass productivity Key words: Crassulacean acid metabolism, productivity, root elongation rate, root system, water uptake     

Root Development and Aerenchyma Formation in Two Wheat Cultivars and One Triticale Cultivar Grown in Stagnant Agar and Aerated Nutrient Solution

Stagnant nutrient solution containing 0.1% agar and with anextremely low oxygen level (‘stagnant agar solution’)was used to simulate the gaseous composition and slow gas diffusionof waterlogged soils. Comparisons were made between the growthof two wheat cultivars(Triticum aestivum,cvs. Gamenya and Kite)and one triticale cultivar(Triticosecale,cv. Muir) grown instagnant relative to aerated solution. For all genotypes tested,immersion of roots in stagnant agar solution resulted in thedeath of the entire seminal root system and led to profuse branchingof the laterals of the nodal roots. In the stagnant agar solutionaerenchyma, as a percentage of the total cross sectional areaof nodal roots, was 18% for Muir, 14% for Kite and 12% for Gamenya;the roots of species with more aerenchyma also attained a longermaximum root length as predicted by the model of Armstrong (in:Woolhouse HW, ed.Advances in botanical research, vol. 7. London:Academic Press, 1979). Muir also had a nodal root/shoot freshweight ratio of 0.5 compared with 0.2–0.3 in Kite andGamenya. The greater number and length of nodal roots of Muirdid not lead to better shoot growth than in the other genotypes;one possible reason for this lack of improvement is a low efficiencyof aerenchymatous roots in wheat.Copyright 1998 Annals of BotanyCompany Root development; aerenchyma; stagnant agar;Triticum aestivumcv. Gamenya;Triticum aestivumcv. Kite;Triticosecalecv. Muir.     

Aerenchyma and an Inducible Barrier to Radial Oxygen Loss Facilitate Root Aeration in Upland, Paddy and Deep-water Rice (Oryza sativa L.)

The present study evaluated waterlogging tolerance, root porosityand radial O₂ loss (ROL) from the adventitious roots, of sevenupland, three paddy, and two deep-water genotypes of rice (Oryzasativa L.). Upland types, with the exception of one genotype,were as tolerant of 30 d soil waterlogging as the paddyand deep-water types. In all but one of the 12 genotypes, thenumber of adventitious roots per stem increased for plants grownin waterlogged, compared with drained, soil. When grown in stagnantdeoxygenated nutrient solution, genotypic variation was evidentfor root porosity and rates of ROL, but there was no overalldifference between plants from the three cultural types. Adventitiousroot porosity increased from 20–26 % for plants grownin aerated solution to 29–41 % for plants grown instagnant solution. Growth in stagnant solution also induceda ‘tight’ barrier to ROL in the basal regions ofadventitious roots of five of the seven upland types, all threepaddy types, and the two deep-water types. The enhanced porosityprovided a low resistance pathway for O₂ movement to the roottip, and the barrier to ROL in basal zones would have furtherenhanced longitudinal O₂ diffusion towards the apex, by diminishinglosses to the rhizosphere. The plasticity in root physiology,as described above, presumably contributes to the ability ofrice to grow in diverse environments that differ markedly insoil waterlogging, such as drained upland soils as well as waterloggedpaddy fields.     

The Use of Agar Nutrient Solution to Simulate Lack of Convection in Waterlogged Soils

Agar at 0.1% in nutrient solution (‘stagnant solution’)was used to prevent turbulence (convection), thus simulatingthe slow gas movements which occur in waterlogged soils. Wheat,aged between 6 and 16 d at the start of the treatment, was usedto test plant growth and development in this stagnant solutionfor 8–15 d. K-MES buffer at 5 mol m^-3was used to retainthe pH of the rhizosphere in the stagnant solution at pH 6.5. The prevention of convection reduced dissolved oxygen concentrationsin the bulk solution from 0.275 to below 0.05 mol m^-3after 1d, while ethylene accumulated over 10 d to 6.5x10^-6m³m^-3(ppm). Aerenchyma of nodal roots grown in stagnant solution comprised22% of the cross sectional area of the root 50 mm behind theroot tip; this was similar to values recorded earlier for nodalroots of wheat in waterlogged soil and contrasts with 7.6% forroots in non-flushed solution without agar (referred to in thispaper as ‘semi-stagnant solution’) and 2.4% in N₂-flushedsolution. Increases in dry weight and numbers of nodal roots with timewere larger for stagnant and N₂-flushed, than for semi-stagnantor aerated solution. In contrast, seminal roots did not growin stagnant solution, while seminal roots in N₂-flushed solutiongrew much less than in semi-stagnant or aerated solution. In the stagnant solution, relatively high concentrations ofN, K and P were required to avoid limitations in mineral uptakeinto the roots, due to the long diffusion pathway from the bulksolution imposed by the lack of convection. Nevertheless, ourdata show that the slow growth imposed by the lack of convectionwas due to factors other than low mineral nutrition. The mostlikely cause was the change in the dissolved gas compositionof the root media, particularly of the rhizosphere. In conclusion, in terms of anatomy and morphology the rootsgrown in the stagnant solution more closely resembled thosefrom waterlogged soil than did those grown in either semi-stagnantor N₂-flushed solution. Triticum aestivum; wheat; waterlogging; lack of convection; aerenchyma; root development; nutrient uptake     

Root Hydraulic Conductivity of Two Cactus Species in Relation to Root Age, Temperature, and Soil Water Status

The effects of root age, temperature, and soil water statuson root hydraulic conductivity (L_P) were investigated for twocactus species, Ferocactus acanthodes and Opuntia ficus-indica.The volumetric flux density of water was measured for excisedroot segments, either using negative hydrostatic pressures appliedto the proximal end or using reverse flow of water from theroot to the soil. For both species, L_P at 20 ?C increased withroot age, average values reaching a maximum of 3.9 ? 10^–7m s^–1 MPa^–1 for F. acanthodes and 5.2 ? 10^–7m s^–1 MPa^–1 for O.ficus-indica at 11 to 17 weeksof age; L_P subsequently declined with increasing root age forboth species. L_P was maximal at a temperature of about 10 ?Cfor the youngest roots (1–3 weeks), this optimum shiftingto 40 ?C for 8-week-old roots of both species. For older roots(up to 1.5-years-old), L_P increased with temperature from 0?C to 50 ?C, with a Q₁₀ of 1.3 between 20 ?C and 30 ?C. At asoil water potential (_soil) of –0.016 MPa, root L_P wasindependent of the direction of water flow for both species.Depending on root age, L_P declined 45- to 500-fold for F. acanthodesand 90- to 800-fold for O.ficus-indica as _soil was reduced from–0.016 to –1.06 MPa, consistent with a rectifier-likebehaviour with respect to water movement between soil and roots.Incorporation of such responses into water uptake models shouldlead to a better understanding of root function. Key words: Ferocactus acanthodes, Opuntia ficus-indica, water potential, tension, reverse flow     

Constraints of root response to waterlogging in Alisma triviale

To understand the economics of root aerenchyma formation in wetland plants, we investigated in detail the response of Alisma triviale to waterlogging. We hypothesized costs being associated with development of a large root air space. In three out-door pot experiments, seedlings (1 experiment) and mature plants (2 experiments) were grown under waterlogged and drained conditions for up to 2?months. Waterlogging promoted growth, and was associated with increased root porosity and decreased root density (fresh mass per volume). The increased formation of aerenchyma was associated with a higher root dry matter content for a given root density. Despite improved growth and earlier flowering, the waterlogged plants also showed signs of being constrained by the anoxic substrate, such as shallower roots, and a higher leaf dry matter content. The formation of aerenchyma was associated with costs, such as increased root dry matter content and reduced metaxylem vessel diameter. The faster growth of the seedlings under the waterlogged conditions, despite some signs of being stressed, was possibly a result of decreased requirements to allocate biomass below ground. In mature plants the increased aerenchyma allowed deeper root penetration, and ameliorated the effects of anoxia, reducing the differences in plant traits between the treatments.     

Distribution of Lateral Root Primordia in Root Tips of Musa

The distribution of lateral root primordia in the root tipsof four Musa landraces (Grande Naine, Pisang Berlin, Ngok Egomeand Yangambi Km5) grown in the field has been investigated toevaluate the range of genetic variation of lateral root initiation.In banana (Musa sp.), lateral roots are initiated in the roottip, 0.6–4 mm behind the root/cap junction and arise inseveral protoxylem-based longitudinal rows or ‘ranks’.Significant differences were observed among landraces for theposition of the most distal primordium, however the longitudinalspacing between successive primordia along the ranks was similarfor all landraces. All ranks were involved in lateral root initiation.The number of ranks also showed significant variations amonglandraces and was proportional to the stelar diameter. Hencethe density of lateral roots (roots cm^-1) was affected by stelardiameter variations. Finally, root elongation in the root tipwas landrace-specific and not necessarily exponential, unlikesuggested in previous studies. It is concluded that lateralroot initiation in Musa is not involved in the genetic variationsof root architecture in the field. A dissection of root architectureinto components which may account for these variations is proposedin relation to the improvement of root system architecture.Copyright 1999 Annals of Botany Company Lateral root initiation, root architecture, Musa, banana.     

Relationship between mineral coating on roots and yield performance of wheat under waterlogging stress

Waterlogging stress limits the productivity of winter wheat(Triticum aestivum L. emend. Thell.) in many parts of the world.When wheat is grown under waterlogged conditions, a reddish-brownmineral coating can form on the epidermal surface of the roots.In wetland plants such as rice, the amount of mineral coatingformed on root surfaces is positively related to yield. Thisstudy was conducted to determine whether mineral coating onthe roots of wheat is related to yield potential under waterloggedconditions. Root mineral coatings formed under waterlogged conditionswere studied in 12 cultivars and two breeding lines over threeyears of greenhouse pot studies. Soil redox potential in thewaterlogged treatment ranged between - 46 and 171 mV, and grainyield was suppressed by 28–49% compared to well-drainedcontrols. Mineral coating formed on the roots from the waterloggedtreatment was determined to be composed primarily of iron, basedon ICP elemental analysis, iron-specific staining, and ion-mappingby scanning electron microscopy using an X-ray detector. Of11 elements quantified by ICP spectroscopy, six were significantlyaffected by waterlogging treatment, and three of these, Fe,Mn and P, were well-correlated negatively to yield. Aerenchymaformation in the heavily coated waterlogged roots appeared todisrupt the internal root structure, and exceeded 40% of cross-sectionalarea in one cultivar. Unlike rice, which shows a positive relationshipbetween oxygen release from roots, grain yield and mineral coating,in winter wheat, the amount of mineral coating is negativelyrelated to grain yield under waterlogged conditions. Key words: Wheat, mineral coating, waterlogging, aerenchyma, hypoxia     

Differential Inhibition of Shootvs.Root Growth at Low Temperature and its Relationship with Carbohydrate Accumulation in Different Wheat Cultivars

Shoot and root growth rate, carbohydrate accumulation (includingfructan), reducing sugar content and dry matter percentage weremeasured in six wheat cultivars, ranging from winter to springtypes, grown at either 5 or 25 °C. At 5 °C (comparedwith 25 °C), the relative growth rate (RGR) of shoots wassimilarly reduced in all cultivars, but the RGR of shoots wasmore affected in winter wheats. This difference resulted insmaller root:shoot ratios than in spring wheats, which alsodeveloped more first-order lateral roots. A direct relationshipbetween carbohydrate accumulation at low temperatures and reductionin root growth was established. These results suggest that differentialshootvs.root growth inhibition at low temperature may play akey role in carbohydrate accumulation at chilling temperatures.This differential response might lead to improvements in survivalat temperatures below 0 °C, regrowth during spring, andwater and nutrient absorption at low temperatures.Copyright1997 Annals of Botany Company Wheat; Triticum aestivum; low temperatures; root growth; root: shoot ratio; sugar accumulation     

Root Respiration for Agave deserti: Influence of Temperature, Water Status and Root Age on Daily Patterns

Root respiration, measured as CO₂ efflux, was studied for asucculent perennial from the Sonoran Desert, Agave deserti,with a new technique using individual, attached roots. The dailypatterns of root respiration closely followed the daily patternsof root temperature for both established roots and rain roots,with higher rates during the day when root temperature averaged27?C and lower rates at night when root temperature averaged17?C. When root temperature was raised from 5?C to 40?C, rootrespiration increased about 7-fold; from 45 ?C to 55 ?C, rootrespiration decreased about 2-fold, except for old establishedroots. Root respiration per unit dry weight for both root typesdecreased with age, the initial decrease being greater for rainroots than for established roots. Root respiration rates forrain roots were reduced to zero at a soil water potential (_soil)of –0.9 MPa and did not recover upon rewatering. Upondrying, root respiration rates for established roots were maintainedat about 12% of maximum, even when _soil fell to –1.6 MPa,and fully recovered 1.5 d after rewatering the soil. Such responsesof rain and established roots must be taken into account whenassessing the carbon costs for the root system. Key words: Agave deserti, CO₂ exchange, root respiration, temperature, soil water potential     

Site, Scale and Time-course for Adjustments in Lateral Root Initiation in Wheat Following Changes in C and N Supply

The pattern of lateral root initiation in seminal roots of wheat(Triticum aestivumL. cv. Alexandria) and the location, scaleand time-course for adjustments in initiation were studied afterchanges in C and N supply. Macroscopically visible primordiaappeared in a non-acropetal sequence with the frequency (numberper unit length) increasing with distance behind the main rootapex to a maximum at 40–50 mm behind the root tip. Pruningthe root system to a single seminal axis increased the primordiafrequency by 23% within 15 h. After longer periods, the effectof root-pruning was greater. The enhanced primordia frequencywas first observed in tissue located 0–10 mm behind theapex at the start of treatment. Feeding glucose (50 mM) alsoincreased primordia frequency within 15 h, but to a greaterextent, and here additional primordia were initiated in tissuelocated 0–10and10–20 mm behind the apex at the startof treatment. Withdrawing NO₃^-from one part of a split-rootsystem, whilst maintaining the supply to the other, reducedprimordia frequency in the non-fed roots and, in some cases,a compensatory increase in the NO₃^--fed roots was observed.The location and scale of the adjustments were similar to thosefound with root-pruning and glucose-feeding, but were slightlyslower to appear. In spite of some differences in detail, therewas a broad similarity in site, scale and time-course for adjustmentsin lateral root initiation with these treatments, which is consistentwith the operation of a common mechanism. Whenever an increasein primordia frequency was observed, it was associated withan increase in the ethanol-soluble sugar content of the tissue.However, the reduction in frequency in NO₃^--deprived roots wasalso accompanied by an increase in sugar content. There wasno consistent relationship between total N content of the tissueand primordia frequency, but there was between primordia frequencyand the rate of net NO₃^-uptake. The possible mechanisms controllinglateral root initiation are discussed. Compensatory growth; correlative growth; glucose; initiation; lateral root; nitrate; primordium; split-root; Triticum aestivum; wheat     

Comparisons of the Respiratory Effluxes of Nodules and Roots in Six Temperate Legumes

The specific respiration rates of nodulated root systems, ofnodules and of roots were determined during active nitrogenfixation in soya bean, navy bean, pea, lucerne, red clover andwhite clover, by measurements on whole plants before and afterthe removal of nodule populations. Similar measurements weremade on comparable populations of the six legumes, lacking nodulesbut receiving abundant nitrate-nitrogen, to determine the specificrespiration of their roots. All plants were grown in a controlled-environmentclimate which fostered rapid growth. The specific respiration rates of nodulated root systems ofthe three grain and three forage legumes during a 7–14-dayperiod of vegetative growth varied between 10 and 17 mg CO₂g^–1 (dry weight) h^–1. This mean value consistedof two components: a specific root respiration rate of 6–9mg CO₂ g^–1 h^–1 and a specific nodule respirationrate of 22–46 mg CO₂ g^–1 h^–1. Nodule respirationaccounted for 42–70 per cent of nodulated root respiration;nodule weight accounted for 12–40 per cent of nodulatedroot weight. The specific respiration rates of roots lackingnodules and utilizing nitrate nitrogen were generally 20–30per cent greater than the equivalent rates of roots from nodulatedplants. The measured respiratory effluxes are discussed in thecontext of nitrogen nitrogen fixation, nitrate assimilation. Glycine max, Phaseolus vulgaris, Pisum sativum, Medicago sativa, Trifolium pratense, Trifolium repens, soya bean, navy bean, pea, lucerne, red clover, white clover, nodule respiration, root respiration, fixation, nitrate assimilation     

Effects of Root Temperature and Form of Nitrogen Nutrition on Nitrate Reductase Activity in Oilseed Rape (Brassica napus L.)

The effect of root temperature and form of inorganic nitrogensupply on in vitro nitrate reductase activity (NRA) was studiedin oilseed rape (Brassica napus L. cv. bien venu). Plants weregrown initially in flowing nutrient solution containing 10 µMNH₄NO₃ and then supplied with either nitrate or ammonium for15 d at root temperatures of 3, 7, 11 or 17 °C. Shoot temperatureregime was similar for all plants; 20/15 °C, day/night.Root NRA was highest when roots were grown at 3 and 7 °C.In laminae and petioles NRA was highest when roots were 11 or17 °C. The plants supplied with ammonium had much lowerlevels of NRA in roots after 5 d than the plants supplied onlywith nitrate. NRA in the laminae of plants supplied with ammoniumwas low relative to that in plants supplied with nitrate onlywhen root temperature was 11 or 17 °C. Values of the apparent activation energy (E_a) of NR, calculatedfrom the Arrhenius equation, in laminae and petioles were differentfrom roots suggesting difference in enzyme conformation. Evidencethat the temperature at which roots were growing affected E_awas equivocal. Oilseed rape, Brassica napus L., activation energy, ammonium, Arrhenius equation, nitrate, root temperature, nitrate reductase     

Growth and Preferences for Ammonium or Nitrate Uptake by Barley in Relation to Root Termperature

Barley plants (Hordewn vulgare L. cv. Atem) were grown fromseed for 28 d in flowing solution culture, during which timeroot temperature was lowered decrementally to 5?C. Plants werethen subjected to root temperatures of 3, 5, 7, 9, 11, 13, 17or 25 ?C, with common air temperature of 25/15 ?C (day/night).Changes in growth, plant total N, and NO₃^– levels, andnet uptake of NH₄⁺ and NO₃^– from a maintained concentrationof 10 mmol m^–3 NH₄NO₃ were measured over 14 d. Dry matterproduction increased 6-fold with increasing root temperaturebetween 3–25 ?C. The growth response was biphasic followingan increase in root temperature. Phase I, lasting about 5 d,was characterized by high root specific growth rates relativeto those of the shoot, particularly on a fresh weight basis.During Phase I the shoot dry weight specific growth rates wereinversely related to root temperature between 3–13 ?C.Phase 2, from 5–14 d, was characterized by the approachtowards, and/or attainment of, balanced exponential growth betweenshoots and roots. Concentrations of total N in plant dry matterincreased with root temperature between 3–25 ?C, moreso in the shoots than roots and most acutely in the youngestfully expanded leaf (2?l–6?9% N). When N contents wereexpressed on a tissue fresh weight basis the variation withtemperature lessened and the highest concentration in the shootwas at 11 ?C. Uptake of N increased with root temperature, andat all temperatures uptake of NH₄⁺, exceeded that of NO₃^–,irrespective of time. The proportions of total N uptake over14 d absorbed in the form of NH₄⁺ were (%): 86, 91, 75, 77,76, 73, 77, and 80, respectively, at 3, 5, 7, 9, Il, 13, 17,and 25 ?C. At all temperatures the preference for NH₄⁺ overNO₃^– uptake increased with time. An inverse relationshipbetween root temperature (3–11 ?C) and the uptake of NH₄⁺as a proportion of total N uptake was apparent during PhaseI. The possible mechanisms by which root temperature limitsgrowth and influences N uptake are discussed. Key words: Hordeum vulgare, root temperature, ammonium, nitrate, ion uptake, growth rate     

Proliferation of Maize (Zea mays L.) Roots in Response to Localized Supply of Nitrate

Maize (Zea mays L.) plants with two primary nodal root axeswere grown for 8 d in flowing nutrient culture with each axisindependently supplied with . Dry matter accumulation by roots was similar whether 1.0 mol m^–3 was supplied to on( or both axes. When was supplied to only one axis, however, accumulationof dry matter within the root system was significantly greaterin the axis supplied with . The increased dry matter accumulation by the +N-treated axis was attributableentirely to increased density and growth of lateral branchesand not to a difference in growth of the primary axis. Proliferation of lateral branches for the + N axis was associatedwith the capacity for in situ reduction and utilization of aportion of the absorbed , especially in the apical region where lateral primordia are initiated. Althoughreduced nitrogen was translocated to the –N axis, concentrationsin the –N axis remained significantly lower than in the+N axis. The concentratio of reduced nitrogen, as well as invitro reductase activity, was greater in apical than in more basal regions of the +N axis. The enhancedproliferation of lateral branches in the + N axis was accompaniedby an increase in total respiration rate of the axis. Part ofthe increased respiration was attributable to increased massof roots. The specific respiration rate (umol CO₂ exolved perhour per gram root dry weight) was also greater for the +N thanfor the –N axis. If respiration rate is taken as representativeof sink demand, stimulation of initiation and growth of lateralsby in situ utilization of a localized exogenous supply of establishes an increased sink demand through enhancedmetabolic activity and the increased partitioning of assimilatesto the + N axis responds to the difference in sink demand between+N and –N axes. Key words: NO₃^- reduction, NO₃^- uptake nitrogen partitioning, root respiration, sink demand     

External Potassium Supply is not Required for Root Growth in Saline Conditions: Experiments with Ricinus communis L. Grown in a Reciprocal Split-Root System

Ricinus communis L. (castor bean) plants were grown in the absence(control) and in the presence of 100molm^–3NaCl with areciprocal split-root system, in which K⁺ was supplied to oneand NO₃^– to the other part of the root system. In theseplants shoot and, to a lesser extent, total root growth wereinhibited compared to plants with non-split roots. Without andwith NaCl, growth of roots receiving NO₃^– but noK⁺ (‘minusK/plus N-roots’) was substantially more vigorous thanunder the reverse conditions (‘plus K/minus N-roots¹).100mol m^–3 NaCl inhibited growth of minus K/plus N-roots¹to the same extent as that of non-split roots, indicating thatexternally supplied K⁺ was not required for root growth undersaline conditions. In growth media without added K⁺ the rootdepleted the external low K ⁺ levels resulting from chemicalsdown to a minimum value C_mln (1.0 to 1.4 mmol m^–3); inthe presence of 100 mol m^–3 NaCl, C_min, was higher (10–18mmol m^–3) and resulted from an initial net loss of K ⁺.C_min, was pH-dependent The distribution of K⁺, Na⁺ and Mg²⁺along the root was measured. In meristematic root tissues, K⁺ concentrations were scarcely affected by external K⁺ or byNaCl, where Na ⁺ concentrations were low, but somewhat elevatedat low external K+ and/or high NaCl. In differentiated, vacuolatedtissues K ⁺ concentrations were low and Na⁺ concentrations high,if K ⁺ was not supplied externally and/or NaCl was present.The longitudinal distribution of ions within the root was usedto estimate cytoplasmic and vacuolar ion concentrations. Thesedata showed a narrow homoeostasis of cytoplasmic K+ concentrations(100–140 mol m^–3) independent of external K ⁺ supplyeven in the presence of 100 mol m ^–3 NaCl. CytoplasmicNa ⁺ concentrations were maintained at remarkably low levels.Hence, external K⁺ concentrations above C_min, were not requiredfor maintaining K/Na selectivity, i.e. for controlling Na⁺ entry.The results are discussed with regard to mechanisms of K/Naselectivity and to the importance of phloem import of K⁺ forsalt tolerance of roots and for cytoplasmic K⁺ homoeostasis. Key words: Ricinus communis, nitrate, potassium, root (split-root), salt tolerance, phloem transport     

Plant growth, nutrient acquisition and mycorrhizal symbioses of a waterlogging tolerant legume (Lotus glaber Mill.) in a saline-sodic soil

Seedlings of Lotus glaberMill., were grown in a native saline-sodic soil in a greenhouse for 50 days and then subjected to waterlogging for an additional period of 40 days. The effect of soil waterlogging was evaluated by measuring plant growth allocation, mineral nutrition and soil chemical properties. Rhizobiumnodules and mycorrhizal colonisation in L. glaberroots were measured before and after waterlogging. Compared to control plants, waterlogged plants had decreased root/shoot ratio, lower number of stems per plant, lower specific root length and less allocation of P and N to roots. Waterlogged plants showed increased N and P concentrations in plant tissues, larger root crown diameter and longer internodes. Available N and P and organic P, pH and amorphous iron increased in waterlogged soil, but total N, EC and exchangeable sodium were not changed. Soil waterlogging decreased root length colonised by arbuscular mycorrhizal (AM) fungi, arbuscular colonisation and number of entry points per unit of root length colonised. Waterlogging also increased vesicle colonisation and Rhizobium nodules on roots. AM fungal spore density was lower at the end of the experiment in non-waterlogged soil but was not reduced under waterlogging. The results indicate that L. glaber can grow, become nodulated by Rhizobium and colonised by mycorrhizas under waterlogged condition. The responses of L. glaber may be related its ability to form aerenchyma.     

Effects of a Range of O2 Concentrations on Porosity of Barley Roots and on their Sugar and Protein Concentrations

Barley was grown at a range of oxygen concentrations (0.5–9mg l^–1), in nutrient solutions. Growth of both shootsand seminal roots was restricted by O₂ concentrations lowerthan 2–3 mg l^–1) but nodal root growth was not. Root porosities were increased even at those O₂ concentrationswhich did not restrict growth, and were inversely proportionalto the protein levels of the roots. Sugar concentrations increasedappreciably only at those O₂ concentrations which also restrictedgrowth. Hordeum vulgare L., barley, root porosity, sugar, protein, oxygen concentration     

Effectiveness of Lotus Root Nodules: III. EFFECT OF COMBINED NITROGEN ON NODULE EFFECTIVENESS AND FLAVOLAN SYNTHESIS IN PLANT ROOTS

When grown in a nutrient solution containing combined nitrogen(NH₄NO₃), Lotus pedunculatus and L. tenuis seedlings inoculatedwith a fast-growing strain of Rhizoblum (NZP2037) did neitherdevelop root nodules nor develop flavolans in their roots. Incontrast, the roots of nodulated seedlings growing in a nitrogen-freenutrient solution contained flavolans. Flavolan synthesis coincidedwith root nodule development on these plants. When added as a single dose, high concentrations of NH₄NO₃ (5and 10 mg N per plant) stimulated the growth of L. pedunculatusplants but suppressed nodulation and nitrogen fixation. In contrastthe continued supply of a low concentration of NH₄NO₃ (1?0 mgN d^–1 per plant) stimulated nitrogen fixation by up to500%. This large increase in nitrogen fixation was associatedwith a large increase in nodule fresh weight per plant, a doublingof nodule nitrogenase activity, and a lowering of the flavolancontent of the plant roots. The close relationship between nitrogendeficiency, nodule development, and flavolan synthesis in L.pedunculatus meant that it was not possible (by nitrogen pretreatmentof plants) to alter the ineffective nodule response of a Rhizobiumstrain (NZP2213) sensitive to the flavolan present in the rootsof this plant.